1. Field of the Invention
The present invention relates to a thin brushless motor.
2. Description of the Related Art
FIG. 6 shows a cross section of a structure of a fan motor that is one example of a conventional brushless motor.
The fan motor includes a cylindrical casing A. A bracket B is integrally formed on a central portion of one end of the casing A on its opening side through a plurality of radial arms. A cylindrical bearing holder C is fitted into a central portion of the bracket B.
A pair of bearings are held on an inner peripheral portion of the bearing holder C. A shaft F which is a center of a rotation portion is fitted into the pair of bearings. The rotation portion is supported by the bearings such that the rotation portion can rotate around the shaft F. A cup-like yoke K is fixed to an upper portion of the shaft F. In impeller H is fixed to an outer peripheral surface of the yoke K. A rotary magnet N is fixed to an inner peripheral surface of the yoke K. The rotary magnet N is opposed to a stator P through a gap in the radial direction. The stator P is fixed to an outer peripheral surface of the bearing holder C.
The stator P comprises a stator core having teeth and a winding wound around the teeth together with an insulator. An electronic circuit board Q is fixed to a lower portion (stationary portion) of the stator P. More specifically, the electronic circuit board Q is connected to the insulator such that a predetermined gap is formed between the electronic circuit board Q and the winding O of the stator P. An IC for controlling the fan motor rotation as a drive control circuit, a resistor, a capacitor, and a Hall IC Q1 are mounted on the electronic circuit board Q. The Hall IC Q1 for detecting a rotation position of a rotary magnet N is mounted on the electronic circuit board Q at a position opposed to the rotary magnet N in the axial direction. Predetermined current is allowed to flow to the winding O to operate the winding O and the stator P side as a stationary portion of the brushless motor, and to operate the yoke K and the rotary magnet N side as a rotation portion of the brushless motor. The electronic circuit board Q control the supply of current to the winding O to rotate the yoke K and the rotary magnet N side with respect to the winding O and the stator P side. The winding O and the electronic circuit board Q are connected to each other through a conductor (not shown). A lead wire R is connected to the electronic circuit board Q, the lead wire R is pulled out from the casing A through an arm, and electricity is supplied to the electronic circuit board Q from outside.
The brushless motor is driven by switching the current direction of the winding O of the stator P in synchronization with a magnetic pole position of the rotary magnet N. Therefore, a sensor for detecting the magnetic pole position of the rotary magnet N is required. A required condition of the sensor is to detect the magnetic field strength and polarity of the magnetic pole of the rotary magnet N. The Hall IC Q1 is in the mainstream as the sensor of the brushless motor. The magnitude of the output voltage and direction of the Hall IC Q1 becomes a product of the magnetic flux density and element current and thus, the Hall IC Q1 can detect the magnetic field strength and polarity. Since the output voltage of the Hall IC Q1 is proportional to the magnetic flux density, if the distance between the rotary magnet N and the Hall IC Q1 is small, i.e., if the axial distance S between the rotary magnet N and the Hall IC Q1 is small, the output voltage of the Hall IC Q1 becomes great correspondingly. Therefore, in the brushless motor, in order to precisely detect the magnetic pole position of the rotary magnet N, it is preferable that the rotary magnet N and the Hall IC Q1 are disposed close to each other.
In the conventional brushless motor structure, however, the electronic circuit board Q is disposed away from the winding O of the stator P by a predetermined distance. Thus, the axial distance S between the electronic circuit board Q and the rotary magnet N which is opposed to the stator P in the radial direction is long. Therefore, the magnetic flux density of the rotary magnet N which influences the Hall IC Q1 mounted on the electronic circuit board Q closer to the rotor becomes low, and there is a problem that the magnetic pole position of the rotary magnet N of the Hall IC Q1 cannot be detected satisfactorily.
Further, when the brushless motor is to be reduced in thickness, if the axial distances between the various parts are reduced, the winding O comes close to a surface of the electronic circuit board Q. Thus, the winding O adversely comes into contact with the electronic circuit board Q in some cases, and the height of the motor cannot be reduced. In addition, in the case of the brushless motor of this kind, the number of turns of the winding O of the stator P is determined in accordance with requirement of the motor characteristics. Thus, since the height of the winding is varied in accordance with the number of turns, the gap between the winding O and the electronic circuit board Q is also varied. As a result, the height of the winding is limited, and the freedom degree of the motor design is deteriorated.